Create a GitHub authtoken and grant repo privileges to the token, then:
sudo ./builder msr.example.com/user/repo:tag $GIT_REPO $GIT_REF $GIT_AUTH_TOKEN
For example:
sudo ./builder some.registry.com/admin/hello-word:cooltag https://github.com/squizzi/hackweek-hello-world.git master SUPERSECRETAUTHTOKEN